Lamps that establish an electric discharge through a gas medium as a means of converting electrical energy into light are classified as electric discharge lamps. One such discharge lamp is a high intensity discharge (HID) lamp that may employ DC type signals developed by ballast circuits for its operation commonly in three modes. The three modes, usually provided by off-line electronic ballasts used for DC HID lamps, include a start circuit, a glow circuit, and a run circuit. The glow function is provided by a high DC link voltage or a voltage doubler circuit across the mains, and a separate linear current limitation device, such as a FET or transistor device, which is switched off by a control circuit when the DC HID lamp is in the run mode.
In the first mode, which is the starting mode, a relatively high value of a DC type starting voltage is necessary to be applied across the electrodes of the HID lamp so as to first place the gases within the lamp in a suitable ionization condition to allow striking or initiating a discharge condition of a glow state or mode. The start circuit is usually a high voltage pulsed DC or a high voltage DC source requiring at least one switching element and one inductive storage element, such as a standard or autotransformer type flyback circuit.
The second mode is the glow mode, which is maintained for a predetermined time to allow the electrodes and the gases to be placed into a state that will sustain an arc condition between the electrodes of the lamp for the purpose of warming the electrodes to a thermionic emission state needed for a run mode. The glow function is provided by a high DC link voltage or a voltage doubler circuit across the mains, and a separate linear current limitation device, such as a FET or transistor device, which is switched off by a control circuit when the DC HID lamp is in the run mode.
The third mode is a steady state or run mode. Initially, the run mode is at a minimum voltage, or V.sub.min, condition, wherein the electrodes are in a thermionic emission state, but the column of gas is cool. The V.sub.min state is the lowest voltage in the run mode, and is typically around 20 volts. As the voltage builds up to a run voltage, which can be anywhere from 50 to 140 volts, the arc discharge of the lamp generates a desired light output at a relatively low or moderate voltage which occurs between the electrodes of the lamp. A forward converter can be used to provide the run function for a DC HID lamp. However, a forward converter requires a reset winding, more sophisticated control circuitry, and/or extra switching components to make sure the core resets. Unfortunately, this adds cost and complexity to the converter. Additionally, transformer losses must be dealt with.
U.S. Pat. No. 4,749,913 of Stuermer et al, assigned to the same assignee as the present invention and herein incorporated by reference, discloses a DC ballast circuit for operating a HID lamp in the start, glow, and run modes of operation. Similarly, U.S. Pat. No. 4,574,219 of Davenport et al, assigned to the same assignee as the present invention and herein incorporated by reference, discloses a lighting unit having a ballast circuit and a spot mode electrode that function to eliminate the need of the glow mode so as to provide the related gas discharge lamp with only the start and run modes of operation. While the ballast circuits of both of these patents serve well their desired needs, they have limitations with regard to the acceptable variations of amplitude and frequency parameters of the AC source applied to the ballast circuits.
One of the limitations of the prior art ballast circuits, is that the amplitude of the AC signal applied to the ballast circuit for rectification thereby should be of a sufficiently high value so that the proportionate amount capable of being derived by the rectifying means is sufficient to supply the level of the DC signal needed to operate the HID lamp in the glow and run modes. If the voltage of the applied AC signal drops below this sufficiently high value, the ballast circuit is not able to develop a DC level sufficient to maintain the arc condition of the HID lamp.
Existing HID ballasts for DC arc tubes usually incorporate a high voltage pulse starting technique and rely on a high mains voltage or a voltage doubler circuit to provide the glow function. Existing glow systems require several high voltage transistors and/or other switching devices to provide both the start and the glow functions. Since existing glow systems use linear regulation methods and the voltage across the linear current regulation device may be quite large, depending on the lamp glow and minimum voltages and the time required to go through the glow period. The instantaneous power stresses are high in these limiting devices, and the limiting devices are expensive and bulky. Control functions must be added to limit the power or time of the glow mode event, and to turn off the glow circuit during run mode.
In some existing glow systems, a separate converter circuit can be used to provide the glow function and is typically added to the existing start and run circuits with diodes. Unfortunately, this means that added magnetic storage devices, switching devices, and control circuitry are needed, all of which add to the complexity and cost of the ballast circuitry.
It is seen then that there exists a need for a simplified ballast circuit which overcomes the problems of complexity and expense encountered in the prior art.